Dzhokhar Tsarnaev, 19, is the younger of the two brothers who authorities say planted the bombs near the marathon's finish line and later in the week killed Officer Collier. Tsarnaev is charged with using a weapon of mass destruction. He became a U.S. citizen in 2012.

Tamerlan Tsarnaev, 26, died April 19 from injuries he sustained during a gun battle with police in Watertown, Mass. Among the things investigators are looking into is whether he made contact with Muslim extremists during trips to visit his parents in Russia. The Tsarnaev family, ethnic Chechens, came to the U.S. about a decade ago. But the parents later left the U.S. Tamerlan Tsarnaev was a legal resident of the U.S. who once hoped to be an Olympic boxer.

THE FAMILY

Katherine Russell is Tamerlan Tsarnaev's American-born wife. Her attorney says she knew nothing about the bombings beforehand and that she is cooperating with investigators.

Zubeidat Tsarnaev is the mother of the two suspects. From Dagestan, where she has been living, she has insisted her sons are innocent. It has also been reported that she encouraged Tamerlan Tsarnaev to embrace Islam, and she "vaguely discussed" jihad with him during a 2011 phone conversation that was recorded by Russian authorities.

Anzor Tsarnaev is the suspects' father. He, too, is in Dagestan and has said he does not believe his sons were responsible for the bombings.

Mikhail Allakhverdov (Misha), is an Armenian-born man living in Rhode Island. Some of the Tsarnaev brothers' family members have accused Allakhverdov of encouraging Tamerlan Tsarnaev to embrace radical Islam. Allakhverdov, though, says he never did that and would have tried to stop the bombing plot if he had known about it.

Dias Kadyrbayev and Azamat Tazhayakov of Kazakhstan, and Robel Phillipos of Cambridge, Mass., are former classmates of Dzhokhar Tsarnaev's at the University of Massachusetts Dartmouth. All three are 19 years old. Kadyrbayev and Tazhayakov have been accused of throwing out key evidence — a laptop and empty fireworks canisters — that could link Dzhokhar Tsarnaev to the bombings. Robels is accused of lying to the FBI about what the friends allegedly did. There's much more about the charges against them here.

KEY WITNESSES

David Henneberry is the Watertown, Mass., man who alerted police when he discovered someone hiding in the boat stored in his backyard. That individual turned out to be Dzhokhar Tsarnaev.

"Danny" is the Chinese engineer turned entrepreneur who says he was taken hostage by the suspects late on April 18, when they commandeered his SUV. Taking Danny with them, they went to ATM machines to get cash. When they stopped for fuel at a gas station, he was able to escape. Because he left his cellphone in the vehicle, police were able to quickly track the SUV to Watertown, where the gun battle occurred.